In a cellular radio communications system, the desired radio coverage area is divided into a number of smaller geographical areas called cells. Initially, when subscriber density is low, it is desirable for economic reasons to serve the area with as few cells as possible. The size of the initial cells is limited in the United States by rules of the Federal Communications Commission (FCC) concerning radiated power in relation to antenna height.
The initial cells are typically provided with one or more antennas each of which have a radiation pattern which covers the entire cell. Such cells may be called omni-directional because, in normal terrain, the antenna pattern will usually be non-directional in order to cover the largest area with the least amount of equipment. However, the significant point is not the directionality of the antenna but the fact that the antenna pattern covers the entire cell. Typically, an antenna combining arrangement is provided so that a number of RF (radio frequency) channel assignments may use each antenna.
The basic feature of cellular systems is that by carefully locating cells and assigning channels to cells, the available frequencies may be reused many times throughout the system--thereby promoting efficient use of the radio spectrum. The number of channels which may be assigned to a cell is limited by the FCC allocation of frequencies to the cellular radio telephone communications service and by interference considerations relating to cell geometry, frequency reuse, etc. Thus, there is an upper bound on both cell size and the number of channels which may be assigned to a cell. These limits determine the number of subscribers which may be served by a system having omni-directional cells.
The channels assigned to a cell include voice channels (used for conversations) and control channels (used to establish contact with mobile transceivers and to direct the mobiles to operate on specific voice channels). Although a control channel must be available to every cell in order to permit the cell to "set up" communications with mobile transceivers, control channels carry no conversations and may be considered to be an overhead expense which it is desirable to minimize.
Cellular systems also have receivers (usually called locating receivers) which are capable of measuring the signal strength of the RF signals received from the mobile transceivers. When a mobile transceiver travels between cells, the signal strength is measured to determine which cell coverage area may best serve the mobile. The mobile transceiver is given a command to retune to a new channel in the new cell when a cell other than the one serving it can provide better service. This process is called handoff.
As the number of subscribers using a cellular system increases, a point is reached at which cells handling heavy traffic no longer have sufficient voice channels to handle the traffic during peak usage times. This problem is solved by subdividing a (large omni) cell coverage area into smaller areas. The use of smaller coverage areas allows frequencies to be reused in closer proximity to each other and thus provides more channels in the same overall area. A number of different arrangements have been proposed for subdividing initial large omni-directional cells. Three basic patterns have evolved. Cell splitting, cell sectoring, and cell overlaying.
In cell splitting, one or more new, smaller cells are added between existing cells. Typically, the existing cells must be made smaller and the channel use patterns must be altered to accommodate the new cells. The net effect is that the original coverage area is subdivided into (mostly smaller) areas which may or may not overlap.
In cell sectoring, directional antennas are used to divide the original cell into smaller areas defined by the directional patterns of the antennas.
The technique of overlaying cells provides a smaller cell at the original cell site. The smaller cell may use the same antenna as the original large omni-directional cell, but the mobile service area is limited by some combination of reduced RF power (mobile, base, or both) and handoff boundaries determined by decisions based on signal strength measurements.
Combinations of these three arrangements have also been proposed.
In all of these arrangements, the subdivided cells must be provided with control channels to permit mobile transceivers to access the subdivided cells. In the sectored and cell splitting arrangements, at least one control channel is provided for each subdivision of the original cell.
The reason each subdivided cell is provided with its own control channel is that idle mobile transceivers "self-locate" by automatically tuning to the control channel received at the strongest signal strength (typically, the control channel signal received at the highest amplitude by the mobile is transmitted by the cell in which the mobile is located). When communications is to be established with the mobile, the mobile is already monitoring the control channel of the appropriate cell and is simply directed to tune to a voice channel of that cell.
Thus, these existing arrangements cause a proliferation of control channels which, as noted above, constitute overhead (i.e., require additional equipment and use additional RF channels which might otherwise be used to carry extra calls) which should be minimized if possible.